<!DOCTYPE html PUBLIC "-//IETF//DTD HTML//EN">
<html>
  <head>
    <meta name="generator" content="HTML Tidy, see www.w3.org">
    <title>SRFI 28: Basic Format Strings</title>
    <meta name="author" content="Scott G. Miller">
    <meta name="description" content="Basic Format Strings">
  </head>

  <body>

    <H1>Title</H1>

    SRFI 28: Basic Format Strings 

    <H1>Author</H1>

    Scott G. Miller 

    <H1>Status</H1>

    This SRFI is currently in ``final'' status. To see an explanation of each status that a SRFI can hold, see <a href="http://srfi.schemers.org/srfi-process.html">here</a>.

    You can access the discussion on this SRFI via <a href="http://srfi.schemers.org/srfi-28/mail-archive/maillist.html">the archive of the mailing list</a>.

    <ul>
      <li>Draft: 2002/03/23-2002/05/23</li>
      <li>Final: 2002/06/25
    </ul>

    <H1>Abstract</H1>

    This document specifies Format Strings, a method of
    interpreting a Scheme string which contains a number of escape
    sequences that are replaced with other string data according to
    the semantics of each sequence. 

    <H1>Rationale</H1>

    Many Scheme systems provide access to a function called <tt>
    format.</tt> This function takes as arguments a format string,
    an ordinary Scheme string containing zero or more escape
    sequences, followed zero or more Scheme values. The procedure
    processes the format string and performs string replacement on
    the escape sequences according to the rules for each code. This
    SRFI defines a basic version of <tt>format</tt> which should
    allow portable code to be written using the function without
    much (if any) effort on the part of Scheme implementors. 

    <H1>Specification</H1>

    <b><a name="format"></a><tt>format</tt></b> <tt><i>format-string [obj ...] -&gt;
    string</i></tt> 

    <blockquote>
      <p>Accepts a message template (a Scheme String), and
      processes it, replacing any escape sequences in order with
      one or more characters, the characters themselves dependent
      on the semantics of the escape sequence encountered.</p>

      <p>An escape sequence is a two character sequence in the
      string where the first character is a tilde '~'. Each escape
      code's meaning is as follows:</p>

      <ul>
        <li><tt>~a</tt> The corresponding value is inserted into
        the string as if printed with <tt>display</tt>.</li>

        <li><tt>~s</tt> The corresponding value is inserted into
        the string as if printed with <tt>write</tt>.</li>

        <li><tt>~%</tt> A newline is inserted.</li>

        <li><tt>~~</tt> A tilde '~' is inserted.</li>
      </ul>

      <p><tt>~a</tt> and <tt>~s</tt>, when encountered, require a
      corresponding Scheme value to be present after the format
      string. The values provided as operands are used by the
      escape sequences in order. It is an error if fewer values are
      provided than escape sequences that require them.</p>

      <p><tt>~%</tt> and <tt>~~</tt> require no corresponding
      value.</p>
    </blockquote>

    <h2>Examples</h2>
<pre>
(format "Hello, ~a" "World!")
; =&gt; "Hello, World!"

(format "Error, list is too short: ~s~%" '(one "two" 3))
; =&gt; "Error, list is too short: (one \"two\" 3))"
</pre>

    <H1>Implementation</H1>

    The implementation below requires SRFI-6 (Basic string ports)
    and SRFI-23 (Error reporting mechanism). <br>
     
<pre>
(define format
  (lambda (format-string . objects)
    (let ((buffer (open-output-string)))
      (let loop ((format-list (string-&gt;list format-string))
                 (objects objects))
        (cond ((null? format-list) (get-output-string buffer))
              ((char=? (car format-list) #\~)
               (if (null? (cdr format-list))
                   (error 'format "Incomplete escape sequence")
                   (case (cadr format-list)
                     ((#\a)
                      (if (null? objects)
                          (error 'format "No value for escape sequence")
                          (begin
                            (display (car objects) buffer)
                            (loop (cddr format-list) (cdr objects)))))
	             ((#\s)
                      (if (null? objects)
                          (error 'format "No value for escape sequence")
                          (begin
                            (write (car objects) buffer)
                            (loop (cddr format-list) (cdr objects)))))
                     ((#\%)
                      (newline buffer)
                      (loop (cddr format-list) objects))
                     ((#\~)
                      (write-char #\~ buffer)
                      (loop (cddr format-list) objects))
                     (else
                      (error 'format "Unrecognized escape sequence")))))
              (else (write-char (car format-list) buffer)
                    (loop (cdr format-list) objects)))))))
    
</pre>

    <H1>Copyright</H1>

    <p>Copyright (C) Scott G. Miller (2002). All Rights Reserved.</p>

    <p>
    Permission is hereby granted, free of charge, to any person
    obtaining a copy of this software and associated documentation
    files (the "Software"), to deal in the Software without
    restriction, including without limitation the rights to use, copy,
    modify, merge, publish, distribute, sublicense, and/or sell copies
    of the Software, and to permit persons to whom the Software is
    furnished to do so, subject to the following conditions:
    </p>
    <p>
    The above copyright notice and this permission notice shall be
    included in all copies or substantial portions of the Software.
    </p>
    <p>
    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
    EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
    MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
    NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
    HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
    WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
    DEALINGS IN THE SOFTWARE.
    </p>
    <hr>

    <address>
      Editor: <a href="mailto:srfi-editors@srfi.schemers.org">
      Francisco Solsona</a>
    </address>

    <address>
      Author: <a href="mailto:scgmille@freenetproject.org">Scott G.
      Miller</a>
    </address>
    <!-- Created: Tue Sep 29 19:20:08 EDT 1998 -->
    <!-- Revised: Friday, March 22, 2002  19:20 PST  -->
    <!-- hhmts start -->Last modified: Mon Apr 8 06:54:37 CDT 2002 
    <!-- hhmts end -->
  </body>
</html>

